FIXED… but not solved, got no answers but kept trying and:
Just disable the new “Interchange” interface, go to plugins and disable everything called “Interchange”, this will bring back the normal import menu and by just choosing your skeletal mesh while importing the animation and 1 click, UE 5.5.1 will import the original FBX animation in 1 file without splitting curves and skeletal animation and making 2 useless files.
I tried every other check box option in the new Interchange menu and even created my own animation import profile wich is a feature of this interface, in order to make faster testings, but nothing fixed this problem, you will have to uninstall the new Interchange interface,
can’t mark it as a solution, since this might actually be a bug in UE 5.5
